We propose to discover how eosinophils protect from C. difficile colitis (CDI). Hypotheses: (1) IL- 1? induced by C. difficile toxins promotes colitisby inhibiting IL-25-mediated gut eosinophilia. (2) IL-25-induced eosinophils protect by repair of the intestinal barrier. Human and murine work: Studies in the murine model of CDI and in humans undergoing fecal microbiota transplant (FMT) for CDI are proposed. Significance: C. difficile is the number one hospital-acquired infection in North America and despite antibiotic therapy has a mortality of 10-15%, with relapses in 10-35% of patients. Approach: There are three Specific Aims: Specific Aim 1: Define the mechanism by which C. difficile binary toxin (CDT) primes the inflammasome to induce IL-1? production. Specific Aim 2: Determine whether IL-1? is the mechanism by which the pathologic innate response to C. difficile infection suppresses eosinophils. Specific Aim 3: Explore the mechanism that colonic eosinophils induced by IL-25 protect from CDI. Successful completion will identify how CDT activates a pathologic innate response, and will delineate the mechanism of protection afforded by IL-25-induced eosinophilia. Therapeutic interventions that are complementary to or supplant fecal transplant and next generation probiotics are potential outcomes. Innovative aspects of the proposal include the exploration of a novel process by which bacterial toxins can promote virulence via activation of pathologic IL-1? and suppression of protective IL-25 induced eosinophils. The environment for the work includes the two graduate students who discovered the role of CDT and eosinophils in CDI, and the lab of the Principal Investigator where 25 years of infectious colitis research have been conducted.